Farangs tend to think of people living in Myanmar as Burmese and thus mostly illegal immigrants here who can't speak Thai. But there are 5 million Tai speaking people living in the Shan state just north of Thailand in Myanmar.
Without knowing the history, one would think of the Shan state as just a part of Thailand that was separated for one reason or another, but that's not the case. The Tai people in the Shan state came from China, as did the Thai people, but much later than the Thais, and they didn't pass through Laos or Thailand, they came south from China with the Mongols (Kublai Khan), entered central Burma, and then were tasked with conquering Northeast Burma for the Mongols.
So, even though they originated in the same areas of China as the Thai people, they have no common history, after the Thai departure from China.
Despite that, Thailand conquered the Shan state during WWII, because they could (Britain held colonial oversight over Burma before WWII, but Japan mostly dislodged them during the war). The end of WWII forced Thailand to return the Shan state, and as far as I know, there is no longer any interest in Thailand in reunification.
There is a civil war in Myanmar now, and some rebels express interest in a free Tai state.
Any insights you may have in the Shan state would be appreciated.